Tuning of the lowest excited states in mixed ruthenium(II) polypyridyl complexes having RuN6 cores by the conformation of the ancillary ligand. Emission from a 3ligand-to-ligand-charge-transfer state

Using the dihydrazone of 2,6-diacetylpyridine as L, two Ru(II) complexes of the type [Ru(bpy)2L](ClO4)2·0.5H2O (1) and [Ru(phen)2L](ClO4)2·1.5H2O (2) are synthesized. Their X-ray crystal structures have been determined. The ligand L, which is a potentially tridentate N donor, is found to bind Ru(II) in 1 and 2 in a bidentate mode – one of the two hydrazone arms remains free. The conformation of this unbound fragment is somewhat different in the two complexes. Complex 1 shows a symmetrical absorption band at 437 nm in the electronic spectrum and 2 a broad, unsymmetrical one around 400 nm. These are of charge transfer origin. Analyses of the frontier molecular orbitals pictorially by means of DFT calculations at the BP86/LanL2DZ level reveal that the HOMO in 1 is localized on L giving rise to an L(π) → bpy(π*) charge transfer, i.e. a ligand-to-ligand charge transfer (LLCT). The other complex has a metal based HOMO giving rise to an MLCT band, which is usual in Ru(II)-polypyridyl complexes. Upon excitation at 440 nm, complexes 1 and 2 display emissions at room temperature in acetonitrile with maxima at 608 and 598 nm respectively. The quantum yield of the emission is 0.076 in 1 with a lifetime of 157 ns and that in 2 is 0.067 with a lifetime of 87 ns. It is concluded that the emission in 1 is from a 3LLCT state and that in 2 from a 3MLCT state. Interestingly, when the conformation of L in 1 is theoretically changed to that observed crystallographically in 2, the HOMO becomes metal based. On the other hand, when the conformation of L in 2 is assumed to be that in 1, the HOMO in 2 becomes L based. Thus the conformation adopted by L in the two complexes determines the nature of their lowest excited state. Emission studies at 77 K in acetonitrile–toluene glass indicate that actually the 3LLCT state is in thermal equilibrium with the 3MLCT state.